The normal human ankle is a joint which is constructed to permit movement in flexion and extension, to provide sliding in the anterior-posterior (A-P) plane, to have a rotation capability and to allow for varus or valgus tiltings. Plantar flexion and dorsiflexion are the major movements of the ankle and may be performed through an angular movement of approximately 20.degree. dorsiflexion and 50.degree. plantar flexion. Rotation is the relative movement of the tibia and the talus about their substantially common vertical axis, the normal rotation limit being approximately 6.degree. to either side, an arc of approximately 12.degree..
Disease indications of the ankle joint include severe rheumatoid arthritis, avascular necrosis of the talus, degenerative and post traumatic arthritis. Until quite recently, the diseased ankle joint has been treated by local repair procedures, or by obliteration of the joint by fusion, which, of course, renders the ankle stiff and generally immobile. The desirability of replacement of the ankle has been apparent, the first reported use of a total ankle prosthesis known to applicant having been by Buckholz in 1969, and attempts toward developing satisfactory devices have been made in the meantime by several investigators, including the present applicant. Applicant developed a plafond ankle joint replacement device, described in his U.S. Pat. No. 3,886,599, which allowed motion in all of the planes involved in the normal human ankle. This prosthesis consisted of an articulated two-part prosthesis having a convex-to-convex bearing surface provided by an upper metallic portion having a stem adapted for engagement with a prepared distal end of the tibia, and a lower portion formed of high density polyethylene and having a shank adapted for engagement with and connection to a prepared surface of the talus. The confronting convex-bearing surfaces of the upper and lower components provide a line contact which allows them to slide in bi-plane fashion relative to each other, and also allows relative rotation of the tibia and talus about their substantially common vertical axis.
Although the just-described prosthesis has been relatively successful, analysis of the results in patients in which it has been implanted revealed that it was not entirely satisfactory in some cases. In the implantation of the device, debridement of the malleoli and their opposing talar surfaces was included as a part of the operative procedure. However, some malleolar pain often persisted, particularly in cases presenting severe pre-operative angulation at the ankle joint and gross arthritis changes in the malleoli. Applicant also felt that the fibrous scar which developed between the debrided surfaces resulted in diminution in prosthetic motion. Accordingly, the primary object of the present invention is to provide an improved implantable total ankle prosthesis having the advantageous features of his earlier device and which will minimize post-operative malleolar pain and preserve the motion inherent in the design of the prosthesis.